I have an old switchgear I am installing a 400 Amp fused disconnect in. 150 feet from that switchgear I am installing a 400 Amp panelboard. The incoming service is a 120/240 Delta 3 Phase system. All I need is 3 phase for 2 10 ton roof top AC Units. We have determined we can do this with 150 amps of service. I had the panelboard manufactured so these two breakers for the AC Units is all that can be installed on the high leg. The rest of the panelboard is for general purpose 120/240 single phase loads. What I have done is pull 2 500 kcmil for the single phase and 2 of the phases of the AC to the panel board. I pulled a 4/0 for the high leg and a 4/0 for the neutral( being the maximum unbalanced load will be 200 amps). I am fusing the 500's at 350 Amps and the 4/0 at 200. I also pulled a number 3 thhn conductor for a ground. Is there anything in the code which says I cant do this? I have checked with the city inspectors here and they looked for three days and couldnt find anything and my bosses cant come up with anything. Message me back please with an answer. Thankyou very much, Leamon
Feeder
- Thread starter lrollo
- Start date
- Status
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
Re: Feeder
Are the breakers that feed the AC units 240 straight voltage rated? They can't be 120/240 rated see nec section 240.85
Are the breakers that feed the AC units 240 straight voltage rated? They can't be 120/240 rated see nec section 240.85
david
Senior Member
- Location
- Pennsylvania
Re: Feeder
215.2 Minimum Rating and Size.
(A) Feeders Not More Than 600 Volts.
(1) General. Feeder conductors shall have an ampacity not less than required to supply the load as computed in Parts II, III, and IV of Article 220. The minimum feeder-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.
First I would start by asking you how you sized the feeder? And what I am getting at is the feeder needs to be sized based on the computed load. I would say loads are computed on VA bases. Once you determine the VA you would size a feeder based on the amps it would take to meet the VA computed load. In my thinking the ungrounded conductors would all be the same size because a feeder calculated load is the sum of the loads of the branch circuits supplied. I see this, as the feeder has to be based on the total VA load.
II. Feeders and Services
220.10 General.
The computed load of a feeder or service shall not be less than the sum of the loads on the branch circuits supplied, as determined by Part I of this article, after any applicable demand factors permitted by Parts II, III, or IV have been applied.
215.2 Minimum Rating and Size.
(A) Feeders Not More Than 600 Volts.
(1) General. Feeder conductors shall have an ampacity not less than required to supply the load as computed in Parts II, III, and IV of Article 220. The minimum feeder-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.
First I would start by asking you how you sized the feeder? And what I am getting at is the feeder needs to be sized based on the computed load. I would say loads are computed on VA bases. Once you determine the VA you would size a feeder based on the amps it would take to meet the VA computed load. In my thinking the ungrounded conductors would all be the same size because a feeder calculated load is the sum of the loads of the branch circuits supplied. I see this, as the feeder has to be based on the total VA load.
II. Feeders and Services
220.10 General.
The computed load of a feeder or service shall not be less than the sum of the loads on the branch circuits supplied, as determined by Part I of this article, after any applicable demand factors permitted by Parts II, III, or IV have been applied.
david
Senior Member
- Location
- Pennsylvania
Re: Feeder
Leamon, are you not going about this backwards?
?I have an old switchgear I am installing a 400 Amp fused disconnect in. 150 feet from that switchgear I am installing a 400 Amp panelboard.?
Wouldn?t the maximum unbalance for a feeder supplying a 400- amp panel be 400 amps? Normally wouldn?t you calculate the load that a feeder is going to supply first then size a feeder and panel to handle that load? Wouldn?t you look at the load in this situation as the 400- amp panel?
Another question has to do with the AC units. Are their control transformers in these AC units and if so are they not being supplied by a grounded ?system?. Would not the fact that the transformers are being supplied by a grounded system mandate the need to pull a neutral to these units?
Leamon, are you not going about this backwards?
?I have an old switchgear I am installing a 400 Amp fused disconnect in. 150 feet from that switchgear I am installing a 400 Amp panelboard.?
Wouldn?t the maximum unbalance for a feeder supplying a 400- amp panel be 400 amps? Normally wouldn?t you calculate the load that a feeder is going to supply first then size a feeder and panel to handle that load? Wouldn?t you look at the load in this situation as the 400- amp panel?
Another question has to do with the AC units. Are their control transformers in these AC units and if so are they not being supplied by a grounded ?system?. Would not the fact that the transformers are being supplied by a grounded system mandate the need to pull a neutral to these units?
websparky
Senior Member
- Location
- Cleveland, Ohio
Re: Feeder
Does a 3 phase delta system have a "neutral"?
I was not aware that one could power a panel with a delta transformer and have single phase loads.
Does a 3 phase delta system have a "neutral"?
I was not aware that one could power a panel with a delta transformer and have single phase loads.
Re: Feeder
OK once again I'm showing my age. This is (was) normal. The high leg was normally sized to the sum of the loads it served. This point of the transformer bank had nothing to do with the single phase part of the system. (center grounded C transformer)
Roger
OK once again I'm showing my age. This is (was) normal. The high leg was normally sized to the sum of the loads it served. This point of the transformer bank had nothing to do with the single phase part of the system. (center grounded C transformer)
Roger
charlie
Senior Member
- Location
- Indianapolis
Re: Feeder
Roger, you are correct. All the three phase load is from the three phases coming into the switchgear. The single phase load is supplied from the "lighting" transformer which is center tapped and grounded. The electric utility could also supply residential customers from this transformer. Additionally, the electric utility could feed this with a single "kicker" (a single transformer fed from a differant phase from the "lighter") that would make an open delta bank.
The bottom line is that I think Leamon is correct. Note the comment by Tom Baker for the three phase loads. The single phase loads may be the slash rated breakers.
Roger, you are correct. All the three phase load is from the three phases coming into the switchgear. The single phase load is supplied from the "lighting" transformer which is center tapped and grounded. The electric utility could also supply residential customers from this transformer. Additionally, the electric utility could feed this with a single "kicker" (a single transformer fed from a differant phase from the "lighter") that would make an open delta bank.
The bottom line is that I think Leamon is correct. Note the comment by Tom Baker for the three phase loads. The single phase loads may be the slash rated breakers.
Re: Feeder
A little more information. We got the manufacturers nameplate ratings from the AC Units and the 4/0 would have been sufficient on its own to handle these two units. If we had wanted to, we could have installed two panels, one for just the AC's and one for lighting and pulled one set of (2) 4/0's, a 4/0 neurtal, and a ground to the lighting load and a set of (3) 4/0's and a ground to the AC panel with no neutral. Also we had the panel board built so as no more load could ever be put on the (B) or High phase. This is an old application. We only have enough space in the old switchgear for the one 400 Amp switch. Also, the power company has a 120/240 single phase transformer on the pole feeding this building And a 240 to ground transformer making the high leg. I have never seen a Delta system done this way. I guess it would be considered an open Delta. So, I thankyou Tom Baker for your insight, I will check to be sure the breakers are straight 240 rated. I hope this helps you to determine my being right or wrong, David, with the new info I provided. There will be control transformers in the AC units but they will be 240 primary and 24 volt secondary and wont require a neutral, websparky. Thanks Roger and Charlie for your input and I think Charlie helped me to understand the configuration of the two primary transformers feeding the building. I appreciate all your help, Have a great weekend, Leamon
A little more information. We got the manufacturers nameplate ratings from the AC Units and the 4/0 would have been sufficient on its own to handle these two units. If we had wanted to, we could have installed two panels, one for just the AC's and one for lighting and pulled one set of (2) 4/0's, a 4/0 neurtal, and a ground to the lighting load and a set of (3) 4/0's and a ground to the AC panel with no neutral. Also we had the panel board built so as no more load could ever be put on the (B) or High phase. This is an old application. We only have enough space in the old switchgear for the one 400 Amp switch. Also, the power company has a 120/240 single phase transformer on the pole feeding this building And a 240 to ground transformer making the high leg. I have never seen a Delta system done this way. I guess it would be considered an open Delta. So, I thankyou Tom Baker for your insight, I will check to be sure the breakers are straight 240 rated. I hope this helps you to determine my being right or wrong, David, with the new info I provided. There will be control transformers in the AC units but they will be 240 primary and 24 volt secondary and wont require a neutral, websparky. Thanks Roger and Charlie for your input and I think Charlie helped me to understand the configuration of the two primary transformers feeding the building. I appreciate all your help, Have a great weekend, Leamon
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